As discussed in commonly assigned, co-pending application Ser. No. 461,427 filed on Apr. 16, 1974, now U.S. Pat. No. 3,906,380, issued Sept. 16, 1975 conventional coherent phase demodulators employ a locally generated, fixed reference carrier which is phase-locked to the signal carrier. The output of the demodulator is proportional to the cosine of the phase difference between the modulated signal carrier and the reference carrier. Thus, when receiving a modulated carrier having phasors A and B at 0.degree. and 180.degree., respectively, the output of the modulator consists of two levels, viz., a first level equal to cos 0.degree. and a second level equal to cos 180.degree..
In phase difference modulated carrier systems employing a repeated phase offset, phase incrementing is utilized to provide clocking information. As is described in more detail hereinbelow, in systems of this type having two phase states for data and employing a repeated phase offset of 45.degree. for each baud interval, the demodulator will see eight different absolute phasors or phase states and will produce five output levels. Similarly, for a modulation system of this type having two phase states and a repeated phase change of 30.degree. for each baud interval, the demodulator will see twelve different absolute phase states and a minimum of six output states will be provided for decoding.
Application Ser. No. 461,427, referred to above, concerns a demodulation system such as described wherein repeated phase shifts are introduced in the reference carrier which track the known repeated phase shifts in the modulated carrier so that the effect of the repeated phase shifts on data information is cancelled. More particularly, the repeated phase shifts introduced by the modulator for the purpose of providing clock information are cancelled in the demodulator, which includes a VCO tuned to the unmodulated carrier frequency and an incrementing circuit that produces the cancelling phase shifts referred to. As a result, the number of output states to be decoded is reduced and the noise performance of the receiver improved. More specifically, considering the two examples referred to above, in a modulation system having two phase states and a repeated phase change of 45.degree.. repeated 45.degree. phase shifts would be introduced in the receiver and the output states reduced to two, thereby simplifying the decoding process and improving noise performance by 3db. Similar results are obtained with a modulation system having two phase states for data and a repeated phase change of 30.degree. for each baud interval, the minimum of six output states being reduced to 2. Decoding in this instance can be performed by simply slicing the output levels about the mid-point (Ov) as opposed to employing level comparators for each of the six levels as required in the prior art.